In traditional heterodyne receivers, a desired signal is downconverted from a carrier frequency to an intermediate frequency (IF) by multiplying a receive signal (that contains the desired signal) with a single sinusoidal signal. One significant disadvantage of a traditional heterodyne receiver is that, in addition to the desired signal, an interfering signal is also downconverted to the IF as illustrated in FIG. 1a. In the example of FIG. 1a, the desired signal 105 and the interfering signal 110 contain frequencies approximately equal distant from a local oscillator frequency 115 used to downconvert the desired signal 105. Thus, a downconverted desired signal 120 is located at an IF 125, however, a downconverted interfering signal 130 (caused by downconversion of the interfering signal 110) falls on top of the downconverted desired signal 120 creating undesirable interference.
To reduce or prevent interference between the downconverted interfering signal 130 and the downconverted desired signal 120, the interfering signal 110 should be suppressed prior to downconverting the desired signal 105. In general, for a desired signal 105 containing frequencies in a rangefc−½fs<f<fc+½fs,  (Equation 1)where fc is a center (i.e., carrier) frequency of the desired signal 105 and fs represents a spectral width of the desired signal 105, interfering signals in a frequency range offc−½fs−2fi<f<fc+½fs−2fi  (Equation 2)will interfere with the downconverted desired signal 120, where fi is the IF 125. Interfering signals in the range of frequencies expressed mathematically by Equation 2 (i.e., mirror frequencies) are typically suppressed using a filter having a high pass filter (HPF) or a band pass filter (BPF) response located prior to the mixer. However, the filter, requiring a very high Q, high order, low noise, etc., is difficult to implement unless fi is large such that the mirror frequencies are sufficiently separated from the desired signal 105.
To eliminate the need for a high-order high-Q filter to suppress mirror frequencies prior to downconversion, low-IF receivers are often implemented. In a typical low-IF receiver, the local oscillator frequency 115 used to downconvert the desired signal 105 has a frequency slightly offset (e.g., by a few hundred kilohertz (kHz)) from the carrier frequency fc. In an example, where the local oscillator frequency 115 is slightly lower than the carrier frequency fc, the desired signal 105 and a mirror signal (i.e., a signal containing frequencies falling within the mirror frequencies) are downconverted, and the resulting downconverted signals are no longer superimposed, as illustrated in FIG. 1b. Instead, a downconverted desired signal 120b is situated at positive frequencies very close to direct current (DC), while a downconverted mirror signal 130b (i.e., a downconverted interfering signal) is situated at negative frequencies very close to DC. The downconverted signals can then be filtered using a complex filter (i.e., a filter having a transfer characteristic that is different for positive and negative frequencies) having a band-pass filter response 135 (i.e., a shifted low-pass filter, centered at an IF 125b) to suppress the mirror signal 130b. 
FIG. 2 illustrates a portion of a prior-art example low-IF quadrature receiver 200 that receives a radio frequency (RF) signal 207 provided by an antenna 202 and a RF receiver 205 (containing, e.g., automatic gain control, a low noise amplifier, filters, etc.). To downconvert the RF signal 207, the quadrature receiver 200 includes a mixer 210 and a local oscillator 215. As discussed above, the mixer 210 downconverts the RF signal 207 by multiplying (i.e., mixing) the RF signal 207 with sinusoidal signals 217 provided by the local oscillator 215. The sinusoidal signals 217 provided by the local oscillator 215 include a first sinusoidal signal and a second sinusoidal signal. The second sinusoidal signal is 90 degrees out of phase relative to the first sinusoidal signal. The frequency of the first and second sinusoidal signals is slightly lower than the carrier frequency fc.
As is well known to persons of ordinary skill in the art, the mixer 210 multiplies the RF signal 207 with the first sinusoidal signal to create an in-phase signal 218 (i.e., I), and multiplies the RF signal 207 with the second sinusoidal signal to create a quadrature signal 219 (i.e., Q). To remove an unwanted mirror signal, the quadrature receiver 200 includes a filter 220. The filter 220 includes, among other things, a complex filter (not shown) that realizes a band-pass filter response suitable for suppressing an unwanted mirror signal (situated at negative frequencies). A known method of implementing the complex filter uses a well-known prior-art polyphase filter.
A typical quadrature receiver 200 employs well-known differential signals rather than single-ended signals. For example, the in-phase signal 218 is constructed using a positive signal and a negative signal, where opposite currents are carried on the two signals, rather than being comprised of a single signal referenced to analog ground. The advantages of, and techniques for, using differential signals are well known to persons of ordinary skill in the art. It is assumed in the following discussions, unless stated otherwise, that signals are differential.
Despite the elimination of the high-order high-Q filter, the low-IF quadrature receiver 200 of FIG. 2 has several practical disadvantages. For example, the mirror signal 130b can have a signal level greater than the desired signal 120b, thereby, requiring additional signal headroom support in the mixer 210. In some circumstances, the mirror signal 130b can be 30 decibels (dB) higher than the desired signal 120b, thus, requiring 70 dB of mirror signal suppression to achieve a signal-to-noise ratio (SNR) of 40 dB; and significantly increasing the dynamic range, linearity, and power consumption that must be supported in the mixer 210 and the filter 220. For these, and additional, reasons, design and implementation of the low-IF quadrature receiver 200 of FIG. 2 is often challenging.